Single Molecule Vibrational Spectroscopy: CO Bonding to Edge and

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Single Molecule Vibrational Spectroscopy: CO Bonding to Edge and Terrace Positions on Ag, Au and Pd Islands on NiAl(110) Thomas M. Wallis, Niklas Nilius, and Wilson Ho J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b02149 • Publication Date (Web): 01 Nov 2016 Downloaded from http://pubs.acs.org on November 1, 2016

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The Journal of Physical Chemistry Letters

Single Molecule Vibrational Spectroscopy: CO Bonding to Edge and Terrace Positions on Ag, Au and Pd Islands on NiAl(110)

T. M. Wallis+, N. Nilius∗, and W. Ho Department of Physics and Astronomy and Department of Chemistry, University of California, Irvine, CA 92697-4575

The vibrational properties of single CO molecules adsorbed on nano-sized Ag, Au, and Pd islands on a NiAl(110) surface were studied with a scanning tunneling microscope. The sensitivity of single molecule vibrational spectroscopy to aspects of the local environment is demonstrated by comparative studies of CO-metal bond vibrations at island terrace and island edge sites. Vibrational spectra of single CO molecules adsorbed on Ag, Au, and Pd island terraces showed peaks at 27 meV, 32 meV, and 44 meV, respectively, which are assigned to the hindered rotational mode. CO molecules on Au and Pd island edges, on the other hand, showed blue-shifted hindered rotational modes at 34 meV and 46 meV, respectively. On Au islands, CO molecules showed a strong preference for adsorption on edges while no such preference was observed on Pd.

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Present address: National Institute of Standards and Technology, Boulder, CO 80305. Present address: Carl von Ossietzky University, Institute for Physics, D-26111 Oldenburg, Germany

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Keywords: Single Molecule Vibrational Spectroscopy, Scanning Tunneling Microscope, Inelastic Electron Tunneling Spectroscopy, Metal Island Adsorption, Carbon Monoxide

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The adsorption of carbon monoxide (CO) on transition metal surfaces has been extensively studied in the past using a variety of surface-science techniques.1,2,3 Its fascination arises from its simplicity that enables a mechanistic understanding of the binding process down to the level of individual orbitals. A first model, the famous Blyholder scheme dating back to the 1960s, describes the interaction as a donation / back donation process involving the 5σ / 2π* orbitals of CO and the metal sp and d electronic states.4 Since then, the model has experienced continuous extensions and refinements, stimulated by the introduction of novel synchrotron and laser-based techniques.5 Especially on idealized flat surfaces, our understanding of CO adsorption has therefore reached a very advanced level.6 A different picture arises for CO molecules in less-ordered chemical environments, the exploration of which is still an experimental challenge. In most catalytically relevant applications, the active metal exists in the form of small particles bound to structurally ill-defined oxide supports.2 These technologically relevant adsorption systems offer a much larger variety of CO binding sites, such as low-coordinated edge and corner atoms, interface sites and metal species in unusual oxidation states, most of which are rarely found or completely absent on single crystal surfaces.7-9 Because the vibrational energies of adsorbates generally depend on the adsorption sites, local spectroscopic techniques are required to probe individual CO molecules in their specific environment. Inelastic electron tunneling spectroscopy (IETS) with the scanning tunneling microscope (STM) combines the chemical sensitivity of vibrational spectroscopy with the sub-Ångström spatial resolution of the STM.10,11 The technique has proven its enormous potential in experiments on individual CO molecules,12-14 and carbonyls adsorbed on metal surfaces.15 The respective spectra revealed not only the stretch and hindered rotational energies of isolated CO species, including the expected isotopic shifts, but also disclosed small energy variations due to intermolecular interactions, as induced by neighboring CO or O species.16 In addition to measurement of energy and intensity, the line shape of CO vibrations has been resolved.17 Recently, inelastic tunneling through a CO-terminated STM tip provided unique insights into the internal

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bonding network of a phthalocyanine molecule.18 Only in a single case, STM-IETS has been employed to probe the vibrational properties of CO molecules on MgO-supported Au islands, serving as model system for catalytically relevant metal nanostructures.19 However, this work could not unambiguously demonstrate the nature of vibrational features in the IET spectra, for example by using isotopically labelled CO molecules. In this study, STM-IETS is used to probe the vibrational characteristics of individual CO molecules bound to Ag, Au, and Pd islands on a NiAl(110) surface. Depending on the chemical identity of the ad-metal, the molecules preferentially populate boundary or terrace sites on the islands. The different binding positions stimulate measurable shifts in the vibrational energy of the individual CO species. Specifically, the hindered rotational mode of CO molecules located at the perimeter of Au and Pd islands was found to be blue-shifted by 2 meV with respect to molecules bound to the island interior. The shift is attributed to an increased electronic backdonation into the CO 2π* orbital at low coordinated metal binding sites. Although we focus on metal-supported islands in our study, we expect similar trends to be observable on oxidesupported nanostructures as used in heterogeneous catalysis. A detailed description of the ultrahigh-vacuum, liquid-helium cooled STM used in our experiments is given in Ref. 20. The STM tips were electrochemically etched from polycrystalline W wire and prepared in situ via alternating cycles of Ne+ self-sputtering and annealing. During experiments, the tip was deliberately brought into contact with the NiAl surface in order to refine the geometry of the tip apex. Thus, the chemical identity of the outermost tip atoms was unknown. The NiAl(110) crystal was prepared by alternating cycles of Ne+ sputtering and annealing to 1300 K. Subsequently, small amounts of Ag, Au or Pd atoms (~0.1 monolayer) were deposited from three separate alumina crucibles onto the surface at 300 K, where they aggregated into small metal islands. Directly after deposition, the system was cooled to 11 K and CO was dosed from a pinhole doser in close proximity to the tip-sample junction. Note that

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NiAl(110) shows a notoriously small reactivity towards CO at low temperature, and most of the molecules attached to the ad-islands, leaving the substrate free of adsorbates.21,22 At the outset of each STM-IETS measurement, the tip position was fixed and the STM feedback loop was turned off. Then the second derivative of the tunneling current with respect to the voltage, d2I/dV2, was measured with a lock-in technique. The d2I/dV2 signal was acquired with a modulation frequency of 200-300 Hz and a rms modulation amplitude of 7-10 mV. The vibrational peak positions and relative changes in conductance, ∆σ/σ, were determined by previously published methods.23 Whereas the position of vibrational modes was found to be independent of the actual tip condition, the measured ∆σ/σ value slightly changed for differently prepared tips. Vibrational images were acquired by rastering the tip across the imaging area with the sample bias tuned to a vibrational mode of the system and recording the d2I/dV2 signal with the feedback loop temporary switched off at each pixel. Silver deposition onto the NiAl(110) surface at room temperature resulted in the formation of large, well-ordered Ag islands, as shown in Fig. 1(a). The islands preferentially extend along NiAl[001] and reach lengths of up to 1000 Å. They show a characteristic inner structure, comprising alternating ridges and troughs running parallel to the NiAl[001] direction. According to DFT calculations, silver grows in the form of (110)-oriented bilayer islands, being in registry with the NiAl(110) surface.24 The ripple structure observed in the STM arises from residual strain at the Ag/NiAl interface. After dosing about 0.01 Langmuir CO at cryogenic temperature, homogeneously distributed molecules were detected as round depressions on the Ag islands, as shown in Fig. 1(c). Their negative contrast in the STM is consistent with previous observations of CO on Ag(110).13,16 The exact adsorption position of individual CO molecules could not be determined, as no atomic resolution was achieved on the densely packed Ag layers. Based on earlier experiments, we anticipate however CO binding to Ag top sites.16 The island perimeter turned out to be unim-

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portant for CO adsorption because the Ag islands in our study were relatively large and therefore exhibited a negligible ratio of edge-to-terrace atoms. STM-IETS spectra taken with the tip positioned above a single CO molecule revealed a peak (dip) at 27 mV (-26 mV) that was not observed in spectra recorded on bare Ag islands [Fig. 1(d)]. The associated conductance change, ∆σ/σ, was of the order of 3%. The symmetric location of the peak and dip structure is a characteristic feature of STM-IETS for a vibrational excitation of the CO molecule. The energy of the spectral feature shows good match with the hindered rotational mode of CO and agrees well with previous STM-IETS data for CO on Ag(110)13 (21 mV) and AgCO on NiAl(110)25 (26 mV). Our assignment is corroborated by recent DFT calculations, showing that the excitation cross section of the CO hindered rotation in STM-IETS is about 25-times larger than that of the adjacent CO-metal stretch mode.26 Further support for this assignment comes from the almost perfect correspondence between the CO hindered rotational energy on Cu surfaces determined by local STM-IETS experiments and that determined from spatially averaged techniques such as infrared spectroscopy, atom scattering, and electron energy loss spectroscopy.27-29 No differences in the vibrational energy have been detected for CO molecules adsorbed in different regions of the Ag islands, e.g. along ridges and troughs of the ripple pattern. Higher vibrational modes, e.g. the CO stretch vibration, could not be accessed in the experiment, as the molecules started jumping between adjacent binding sites at a bias voltage above 250 mV. This value is close to the expected energy of the CO stretch vibration and indicates an efficient energy transfer between vibrational and translational degrees of freedom in CO molecules bound to the Ag islands.16 Bare and CO-decorated Au islands show significant differences from their Ag counterparts. Crystalline, single-layer Au islands were observed at the downside of step edges and on large terraces of the NiAl(110) surface [Fig. 2(a)]. Most Au islands exhibit distinct elongated shapes, although their edges are not as straight as the Ag island edges. In topographic STM images, the

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islands display patterns of parallel stripes with about 12 Å periodicity, tilted by ±19 degrees against the NiAl[001] direction. The Au islands thus occur in two mirror domains, clearly distinguishable via their stripe orientation. We tentatively assign this stripe structure to a (1×3) reconstruction of Au(110), a common phase on bulk Au(110).30,31 We do not understand at this point why, in contrast to silver, the Au(110) lattice does not align with the NiAl crystal lattice, but is rotated by ±19°. An earlier DFT and STM work investigated the relative stability of different Au layers on NiAl(110) but did not address the issue of crystallographic orientation.32 Upon CO exposure, single molecules have been detected on both the island edges and terraces. While CO molecules bound to terrace sites appear as sombrero-shaped depressions, those adsorbed on edge sites are imaged as round protrusions [Fig. 2(c)]. Interestingly, the number of CO molecules bound to edge sites largely exceeds the number of molecules on terrace positions, providing evidence for the enhanced reactivity of the Au edge atoms. For example, the Au island shown in Fig. 2(c) contains about 13 edge molecules but only a single terrace molecule. Vibrational spectra taken over individual CO molecules show features that do not appear in the background spectra taken on bare Au islands [Fig. 2(d)]. The corresponding conductance change ∆σ/σ is of the order of 4%, while peak energies are determined to 32 meV (dips -30 meV) and 34 meV (dips -32 meV) for CO adsorbed on terrace and edge positions, respectively (Table 1). These values lie well inside the reported energy range of the CO hindered rotation. For individual gold carbonyls (AuCO) on NiAl(110), for example, a hindered rotational energy of 35 meV has been reported,25 a value that shifts to 37 meV for CO bound to monatomic Au chains.33 Simultaneously obtained topographic and vibrational images of a CO-covered Au island are shown in Figs. 3(a)-(b). The data was taken at 34 mV sample bias, tuned to the energy of the CO hindered rotational mode. Evidently, regions of high d2I/dV2 intensity match the position of the CO molecules in the topographic channel, and both the edge and terrace molecules are clearly discernable. The measurement further demonstrates that the majority of CO decorates the Au edges, while the number of terrace-bound molecules is comparatively small.

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Pd deposition on NiAl(110) also results in the development of flat, elongated metal islands with the long axis aligned with the NiAl[001] direction [Fig. 4(a)]. A large number of randomly arranged protrusions and depressions on the island terraces suggest a higher degree of disorder in the Pd layers. We explain this observation with the large mismatch of bulk Pd and NiAl lattice parameters of 2.75 Å and 2.89 Å, respectively. In contrast, the nearest neighbor distance in Ag and Au crystals (2.89 Å) agrees well with the unit-cell size of NiAl, so that ordered metal ad-layers can be formed. Single CO molecules on Pd islands are imaged as protrusions in the STM, both on edge and terrace sites (Fig. 4(b)). Moreover, edge and terrace species have a similar abundance on islands of about 100 Å length, i.e. the clear preference for edge adsorption as found for gold is not observed here. Given the disordered, non-uniform appearance of the Pd islands in STM topographies, STM-IETS measurements were required to distinguish structural protrusions from actual CO molecules. A vibrational spectrum taken with the tip positioned above a single terrace CO shows a peak (dip) at 44 meV (-42 meV) that is not present in background spectra taken over bare Pd (Fig. 4(c)). We note that also the background exhibits some point-to-point variations; however these are not compatible with inelastic excitations and rather reflect local variations in the Pd electronic structure. The pronounced peak / dip features revealed in all CO spectra is assigned to the hindered rotational mode of the molecule again and shows a ∆σ/σ of 9%. Our conclusion is supported by STM-IETS measurements of PdCO on NiAl(110) that found the hindered rotation at 40 meV,34 but also by non-local electron-energy-loss studies performed for the CO/Pd(110) system.35 Similar to the findings on Au islands, CO molecules adsorbed on Pd edge sites exhibit a vibrational energy that is upshifted by 2 meV with respect to molecules bound to the island terrace (Fig. 4(d)), as shown in Table 1. The relative ordering of measured vibrational energies of individual CO molecules on Ag, Au, and Pd islands mirrors the reactivity of the respective metals, hence the anticipated binding strength of the adsorbates.6,36 Already the classical Blyholder scheme is sufficient to rationalize

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the observed shifts.4 Here, the binding strength is governed by the ability of the CO to donate charges from its 5σ orbital into the metal sp band, combined with the efficiency of electron back-donation from the metal d states into the 2π* orbitals of the molecule. The former process reduces Pauli repulsion between the molecule and the metal surface and is equally efficient for Ag, Au and Pd, all possessing wide, half-filled sp bands crossing the Fermi level. The latter process directly strengthens the interfacial bond and features pronounced differences between the three metals.37 Silver has the lowest d-band onset and is virtually unable to promote electron back-donation into the 2π* CO states, explaining the small binding strength and the low vibrational energy of the molecule observed in STM-IETS. The d bands of Pd, on the other hand, reach the Fermi energy and electron transfer into the CO is easily possible. This is reflected in a comparatively large CO adsorption energy6 and the most blue-shifted hindered rotational mode of all investigated metals. Gold takes an intermediate binding strength and vibrational energy, in good agreement with the position of its d band in between the bands in Ag and Pd. Not surprisingly, the same ordering as for the metal islands has been found for the hindered rotational mode in AgCO, AuCO, and PdCO species on NiAl(110)25,34 and for CO on Ag(110),18 Au(110),17 and Pd(110)35 (Table 1). For Au and Pd islands, a blueshift of the hindered rotational energy for edge versus terrace CO has been found in our study (Table 1). Similar shifts were earlier observed with nonlocal spectroscopic techniques, e. g. HREELS and IRAS, where the spatially averaged vibrational response of CO was probed on flat and vicinal metal surfaces.38-40 In those studies, a preferred occupation of the metal edge sites was revealed, associated with a strongly red-shifted C-O stretch frequency with respect to terrace-bound species.41 This finding fully agrees with the blue-shifted hindered rotational mode of edge CO observed here. A strengthened CO-metal bond and a weakened internal molecular bond both indicate an enhanced interaction of CO with low-coordinated metal atoms located at step edges. Two mechanisms are responsible for this trend, according to the Blyholder model. The energy position of the metal d states upshifts with

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decreasing coordination of the transition metal atoms, reflecting the effect of hybridization and band formation.37 As a result, the back-donation mechanism is more efficient for lowcoordinated edge than terrace atoms. Conversely, the sp electron density is reduced at the edge and corner atoms that protrude from the compact electron gas at the metal surface, which in turn promotes 5σ donation from the CO. Both effects increase the binding potential of lowcoordinated metal atoms towards CO and explain the observed blueshift of the CO hindered rotational mode.4-6 Our study revealed however pronounced differences in the ratio of edge to terrace CO on Au and Pd islands. While more than 90% of the molecules on Au islands bind to the perimeter, this ratio decreases to 50% on Pd islands. A first explanation is based on the overall chemical inactivity of gold with respect to Pd. While Pd(111) binds CO with 1.3 eV, this value drops to 0.04 eV on Au(111).6 In fact, only low-coordinated Au atoms offer considerable bonding to molecular adsorbates at all, an effect that could be verified in this and earlier experiments.40,42 The high structural homogeneity of the Au islands on NiAl(110) further promotes the edge effect in adsorption, because CO diffusion barriers across the well-ordered Au surface are expected to be low. By contrast, the apparent disorder of the Pd islands hinders CO diffusion and offers a large number of CO binding sites even on the island terraces. A previous STM study attributed the growth of disordered Pd islands on Cu(111) to an exchange of Pd and Cu atoms across the interface.43 However, our data revealed only the characteristic vibrational fingerprint of CO on Pd, while no indications for CO-Al or CO-Ni bonds have been found. An explanation of the missing edge effect in the CO/Pd adsorption system with a possible Pd-NiAl interface mixing thus appears unlikely. The vibrational properties of single CO molecules adsorbed on Ag, Au, and Pd islands on NiAl(110) were measured by STM-IETS. The CO hindered rotational mode was found to be blue-shifted by about 2 meV for edge versus terrace-bound molecules on the Au and Pd islands. Moreover, the island edges were preferred for CO bonding in the case of Au islands while no

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such preference was observed for Pd. Our single molecule experiments provided unique insights into local differences in the CO binding behavior to spatially inhomogeneous surfaces. They demonstrated that site-dependent STM-IETS is indeed a powerful tool to specifically probe edge and interface binding sites on catalytically relevant adsorption systems.

AUTHOR INFORMATION Corresponding Author E-mail: [email protected]. Tel.: (949) 824-5234. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is supported by the Condensed Matter Physics Program, Division of Materials Research, National Science Foundation, under grant DMR-1411338 and by a fellowship of the German Science Foundation (NN).

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(39) Yates, J. T.; Surface Chemistry at Metallic Step Defect Sites. J. Vac. Sci. Tech. A 1995, 13, 1359-1367. (40) Ruff, M.; Frey, S.; Gleich, B.; Behm, R. J. Au-Step Atoms as Active Sites for CO Adsorption on Au and Bimetallic Au/Pd(111) Surfaces. Appl. Phys. A 1998, 66, 513-517. (41) Brandt, R. K.; Greenler, R.G. Arrangement of CO Adsorbed on Pt(533). Chem. Phys. Lett. 1994, 221, 219-223. (42) Stiehler, C.; Calaza, F.; Schneider, W.-D.; Nilius, N.; Freund, H. J. Molecular Adsorption Changes the Quantum Structure of Oxide-Supported Gold Nanoparticles: Chemisorption versus Physisorption. Phys. Rev. Lett. 2015, 115, 036804. (43) Aaen, A. B.; Lægsgaard, E.; Ruban, A. V.; Stensgaard, I. Submonolayer Growth of Pd on Cu(111) Studied by Scanning Tunneling Microscopy. Surf. Sci. 1998, 408, 43-56.

Table 1: Summary of STM-IETS data of the CO hindered rotational mode observed on Ag, Au and Pd islands grown on NiAl(110) in comparison with single carbonyl species on the same NiAl(110) surface and for CO on Ag(110) and Au(110) surfaces. The bottom row shows results from an EELS study performed on Pd(110). System

IETS (This Study) Terrace Edges

IETS Other Studies

Ag/NiAl(110)

27 meV

---

26 meV 25

Au/NiAl(110)

32 meV

34 meV

35 meV 25

Pd/NiAl(110)

44 meV

46 meV

40 meV 34

Ag(110)

19 meV18

Au(110)

32 meV17

EELS

42 meV 35

Pd(110)

15



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Figure Captions Figure 1: STM topography and vibrational spectroscopy: CO/Ag islands/NiAl(110). (a) A topographic image shows large, ordered Ag islands on NiAl(110) before the CO dose. The image was acquired at 300 K with 1.06 V sample bias and 0.1 nA tunneling current. Image size is 1000 Å × 1000 Å. (b) A close-up topographic image of an Ag island terrace before the CO dose shows a pattern of ridges and troughs running parallel to the [001] direction. The image was acquired at 0.33 V sample bias and 0.1 nA current. Image size is 125 Å × 125 Å. To improve contrast, an “illuminated” image has been generated by performing a line-by-line differentiation of the topographic data. (c) A topographic image shows individual CO molecules as depressions on Ag islands. The image was acquired at 11 K with a sample bias voltage of 100 mV and a tunneling current of 1.0 nA. Image size is 94 Å × 94 Å. (d) STM-IETS measurements taken with the tip positioned above a CO molecule and above the bare Ag island. Both spectra were acquired with the same tip. The tip-sample distance was fixed with a sample bias voltage of 100 mV and a tunneling current of 0.1 nA. The peak (dip) at 27 meV (-26 meV) is assigned to the hindered rotational mode.

Figure 2. STM topography and vibrational spectroscopy: CO/Au islands/NiAl(110). An illuminated, topographic image shows Au islands on NiAl(110) before the CO dose. The image was acquired at 11 K with 1.88 V sample bias and 0.1 nA tunneling current. Image size is 1000 Å × 1000 Å. The black arrow indicates a long Au island adjacent to the downside of a NiAl step edge. (b) A close-up topographic image before CO dosing shows patterns of parallel rows as well as individual Au adatoms located on top of the monolayer islands. The image was acquired at 11 K with 1.88 V sample bias and 0.1 nA tunneling current. Image size is 250 Å × 250 Å. (c) A topographic image shows an Au island after exposure to CO. The image was acquired at 11 K with a sample bias of 200 mV and a tunneling current of 0.1 nA. Image size is 94 Å × 94 Å. Examples of CO adsorbed at edge (e) and terrace sites (t) are labeled. The inset depicts a close

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up of a single CO molecule adsorbed on a terrace site. (d) STM-IETS measurements taken with the tip positioned above a CO molecule on an edge site (e), above a CO molecule on a terrace site (t) and above the bare Au island. All spectra were acquired with the same tip. The tipsample distance was fixed with a sample bias voltage of -88 mV and a tunneling current of 0.15 nA. The peaks (dips) at 34 meV (-32 meV) and 32 meV (-30 meV) are assigned to the hindered rotational mode. The magnitude difference in the positions of the peaks and dips can arise from a zero-offset in the D/A converter, imprecision in peak determination due to nonzero background and spectral width, and contact potential difference.

Figure 3. Vibrational microscopy: CO/Au islands/NiAl(110). Simultaneously acquired (a) topographic and (b) vibrational images of an Au island with a single CO molecule on the island terrace and a number of CO molecules decorating the island edges. Both images, 66 Å × 66 Å, were acquired at 11 K with a sample bias voltage of 34 mV and tunneling current of 0.10 nA.

Figure 4. STM topography and vibrational spectroscopy: CO/Pd islands/NiAl(110). A topographic image shows Pd islands on NiAl(110) before the CO dose. The image was acquired at 11 K with 2.0 V sample bias and 0.1 nA tunneling current. Image size is 250 Å × 250 Å. (b) Illuminated topographic image of a Pd island after exposure to CO. One CO molecule adsorbed on an edge site (e) and three CO molecules adsorbed on terrace sites (t) are labeled. The image was acquired at 11 K with 98 mV sample bias and 0.1 nA tunneling current. Image size is 83 Å × 83 Å. (c) STM-IETS measurements taken with the tip positioned above a CO molecule on a terrace site and above the bare Pd island. Both spectra were acquired with the same tip. The tipsample distance was fixed with a sample bias voltage of 76 mV and a tunneling current of 0.1 nA. The peak (dip) at 44 meV (-42 meV) is assigned to the hindered rotational mode. (d) STMIETS measurements taken with the tip positioned above a CO adsorbed on an edge site (e) and above a CO adsorbed on a terrace site (t). Both spectra were taken with the same tip, but a dif-

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ferent tip than that used for the measurements shown in (c). The gap was fixed with a sample bias voltage of 105 mV and a tunneling current of 0.14 nA.

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Figure 1

19



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Figure 2

20


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Figure 3

21



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Figure 4

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(a)

(b)

[0 0 1] [1 1 0]

(c)

(d) d2I/dV 2 (nA/V 2)

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27

CO -26

10

CO Low

High

Ag

0

-40

-20

0

20

40

Sample Bias (mV)



(a)

(b)

(c) (d)

[1

1

0

10

e

]

Low

34

e

32

d2I/dV 2 (nA/V 2)

0

1

]

t [0

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High

t

-32

Au

0 -30

-40

0

40

Sample Bias (mV)


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(a)

Low

(b)

High

Low


High


(b)

[0

01

]

(a) [1

10

t

]

e High

(c)

44

-42

0

High

(d)

46

10

t 10

Low

Pd

d2I/dV 2 (nA/V 2)

Low

d2I/dV 2 (nA/V 2)

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5

44

0 -5

e t

-10

-10

-40

0

40

Sample Bias (mV)

0

20

40

60

Sample Bias (mV)


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CO

Au

NiAl


Single Molecule Vibrational Spectroscopy: CO Bonding to Edge and

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